1. Field of the Invention
This invention relates to quasi-optic grid arrays, such as periodic grid arrays, and in particular to techniques for adapting a waveguide to a quasi-optic grid array.
2. Description of Related Art
Broadband communications, radar and other imaging systems require the transmission of radio frequency (xe2x80x9cRFxe2x80x9d) signals in the microwave and millimeter wave bands. In order to efficiently achieve the levels of output transmission power needed for many applications at these high frequencies, a technique called xe2x80x9cpower combiningxe2x80x9d has been employed, whereby the output power of individual components are coupled, or combined, thereby creating a single power output that is greater than an individual component can supply. Conventionally, power combining has used resonant waveguide cavities or transmission-line feed networks. These approaches, however, have a number of shortcomings that become especially apparent at higher frequencies. First, conductor losses in the waveguide walls or transmission lines tend to increase with frequency, eventually limiting the combining efficiency. Second, these resonant waveguide cavities or transmission-line combiners become increasingly difficult to machine as the wavelength gets smaller. Third, in waveguide systems, each device often must be inserted and tuned manually. This is labor-intensive and only practical for a relatively small number of devices.
Several years ago, spatial power combining using xe2x80x9cquasi-opticsxe2x80x9d was proposed as a potential solution to these problems. The theory was that an array of microwave or millimeter-wave solid state sources placed in a resonator could synchronize to the same frequency and phase, and their outputs would combine in free space, minimizing conductor losses. Furthermore, a planar array could be fabricated monolithically and at shorter wavelengths, thereby enabling potentially thousands of devices to be incorporated on a single wafer.
Since then, numerous quasi-optical devices have been developed, including detectors, multipliers, mixers, and phase shifters. These passive devices continue to be the subject of ongoing research. Over the past few years, however, active quasi-optical devices, namely oscillators and amplifiers, have evolved. One benefit of spatial power combining (over other methods) using quasi-optics is that the output power scales linearly with chip area. Thus, the field of active quasi-optics has attracted considerable attention in a short time, and the growth of the field has been explosive.
It is believed that the first quasi-optical grid array amplifier was a grid developed by M. Kim et al. at the California Institute of Technology. This grid used 25 MESFET differential pairs, demonstrating a gain of 11 dB at 3 GHz. As shown in FIG. 1, a typical grid amplifier 10 is an array of closely-spaced differential pairs of transistors 14 on an active grid 12 sandwiched between an input and output polarizer, 18, 24. An input signal 16 passes through the horizontally polarized input polarizer 18 and creates an input beam incident from the left that excites rf currents on the horizontally polarized input antennas 20 of the grid 12. These currents drive the inputs of the transistor pair 14 in the differential mode. The output currents are redirected along the grid""s vertically polarized antennas 22, producing a vertically polarized output beam 30 via an output polarizer 24 to the right.
The cross-polarized input and output affords two important advantages. First, it provides good input-output isolation, reducing the potential for spurious feedback oscillations. Second, the amplifier""s input and output circuits can be independently tuned using metal-strip polarizers, which also confine the beam to the forward direction. Numerous grid amplifiers have since been developed and have proven thus far to have great promise for both military and commercial RF applications and particularly for high frequency, broadband systems that require significant output power levels (e.g.  greater than 5 watts) in a small, preferably monolithic, package. Moreover, a resonator can be used to provide feedback to couple the active devices to form a high power oscillator.
Grids amplifiers can be characterized as quasi-plane wave input, quasi-plane wave output (free space) devices. Grid oscillators are essentially quasi-plane wave output devices. However, most microwave and millimeter wave systems transport signals through electrical waveguides, which are devices that have internal wave-guiding cavities bounded by wave-confining, and typically metal, walls. Consequently, an interface between the two environments is needed in most cases. This interface is needed whether the electric field signal is being output from a waveguide for effective application to the grid array; or the free space output signal of a grid array is to be collected into a waveguide.
Providing such an interface is not a trivial matter for several reasons. First, microwave and millimeter wave waveguides conventionally transmit signals in the single transverse electric (TE) mode, also known as the fundamental, or TE10, mode, and block the higher-order mode components of the signal. These conventional waveguides have a standard, constant size and rectangular shape. However, the input plane area of any typical grid array upon which the input signal is incident may be much larger than the area of the standard rectangular waveguide aperture. Furthermore, as noted, grid array assemblies comprising N by N unit cells and bounded by a dielectric (see FIG. 2) will vary in size depending number of cells in the grid and the dielectric size. Thus, a standard waveguide cannot directly mate with a grid array structure.
Moreover, the standard single mode rectangular waveguide operating in TE10 mode provides an electric field distribution that varies sinusoidally in amplitude across it aperture. However, efficient operation of grid amplifiers requires an excitation beam that has a relatively uniform phase and magnitude distribution across the amplifier""s area.
Several groups have attempted to design waveguides that interface with quasi-optic active devices, but have had only limited success. For example, Yang, et al. recently published an article titled xe2x80x9cA Novel TEM Waveguide using Unipolar Compact Photonic-Bandgap xe2x80x9d, IEEE Trans. On Microwave Theory and Tech., Vol. 48, No. 2, pp. 2092-2098, November, 1999. Further, Ali, et al. published an article titled, xe2x80x9cAnalysis and Measurement of Hard-Horn Feeds for the Excitation of Quasi-Optic Amplifiers,xe2x80x9d IEEE Trans. On Microwave Theory and Tech., Vol. 47, No. 4, pp. 479-487, April, 1999. Unfortunately, these proposed techniques do not adequately resolve the aforementioned problems. For example, the photonic bandgap structures described by Yang et al. are very difficult and costly to manufacture, making this technique less than desirable. Moreover, the xe2x80x9chard-hornxe2x80x9d approach of Ali et al. creates a rather large and bulky structure that is impractical for most commercial applications.
Thus, there is a definite need for a simple and cost effective interface, or adapter, that efficiently couples a waveguide that propagates signals in the fundamental mode to a grid array structure with a desired field distribution,
The present invention, which addresses these needs, resides in an adapter for coupling a quasi-optic grid array assembly to a waveguide that has an internal cavity bounded by a wave-confining device and that guides a wave propagating in a longitudinal direction. The adapter translates the wave between the fundamental mode of the waveguide and a desired electromagnetic field distribution at the plane of the array assembly. The adapter comprises a first end, a second end and a wave-confining structure. The first end that is adapted to mate with an end of the waveguide and that defines a first aperture that substantially matches the size of the waveguide cavity at the end of the waveguide. The second end defines a second aperture that is larger than the first aperture. The wave-confining structure is disposed between the first aperture and second aperture and defines a wave-guiding cavity that guides a wave propagating along the longitudinal direction of signal propagation. The wave-confining structure includes means for creating a spatial discontinuity within the cavity of a predetermined size to create a desired field distribution. In one embodiment, this includes a first step configured within the cavity that is a predetermined distance from the first aperture. The spatial discontinuity is defined as a substantially abrupt change in cross-section of the wave-confining structure, which may simply be internal walls. Although application dependent, typically, the change in cross-section occurs preferably over less than xc2xc of a wavelength. It will be understood by those skilled in the art that other changes (magnitude and shape) in cross-section are possible. Accordingly, the adapter of the present invention tends to be substantially more manufacturable and compact than the conventional techniques and devices.
More particularly, the first step in the internal walls of the adapter adjusts the size of the guiding cavity in the direction parallel to the electric field propagating in the waveguide, referred to herein the xe2x80x9cE-plane.xe2x80x9d Alternatively, the first step may adjust the size of the guiding cavity in the direction perpendicular to both the direction of the electric field and the longitudinal direction of the wave propagation, referred to hereinafter as xe2x80x9cH-plane.xe2x80x9d Preferably, however, the adapter of the present invention in includes at least two steps within the guiding cavity; one that adjusts the cavity size in the E-plane and another that adjusts the cavity size in the H-plane. The adapter of the present invention may further include at least one additional step in the E-plane within the adapter walls and one additional step in the H-plane within the adapter walls. All of these steps are configured to excite higher order modes within the adapter and to shape the field distribution of the signal at the second aperture.
In one preferred embodiment, the adapter of the present invention includes a grid array located at the second aperture of the adapter. This grid array assembly includes an active grid array bounded by a dielectric, which serves as a heat spreader. The grid array may be a grid amplifier, a grid oscillator or other type of active grid array. Moreover, the second aperture is sized such that the edges of the active array are spaced apart from the confining walls a predetermined distance in order to shape the field distribution incident at the second aperture.
In another more detailed aspect of the invention, an input feed device for an active quasi-optic grid array assembly, that expands the fundamental mode of a signal propagating longitudinally in a rectangular waveguide having an internal wave-confining cavity, to a multi-mode signal having a desired field distribution is disclosed. This device comprises: (1) an input defining a first aperture that substantially matches the size of the waveguide cavity and that is adapted to mate with the waveguide; (2) an output defining a second aperture that is adapted to contain the grid array; and (3) a wave-confining structure disposed between the input and output, defining an EM guiding cavity. The wave-confining structure includes a first step within the cavity that is a predetermined distance from the input and that expands the cavity by a predetermined size, thereby controlling the phase and amplitude distribution of the signal between the fundamental mode of the waveguide and higher-order modes to obtain a desired field distribution. The step enlarges the guiding cavity in the E-plane or H-plane. Preferably, however, the feed cavity includes one step that enlarges the cavity in the E-plane and a second step that enlarges the cavity in the H-plane. There may be additional steps within the cavity in order to obtain a desired field distribution at the output.
In yet another embodiment, an electromagnetic wave collector device that translates a multi-mode signal propagating from an active quasi-optic grid array assembly into the fundamental mode of a rectangular waveguide having an internal conducting cavity, is disclosed. The collector includes an input defining a first aperture adapted to contain the grid array, an output defining a second aperture that substantially matches the size of the waveguide cavity and that is adapted to mate with the waveguide, and a wave-conducting structure disposed between the input and output, defining an EM wave-guiding cavity.
The cavity includes first step in the E-plane or H-plane that is a predetermined distance from the input and that contracts the cavity by a predetermined size, thereby controlling the phase and amplitude distribution of the signal in order to convert the power in the higher-order modes of the signal into the fundamental mode from the grid array. More preferably at least two steps are included in the cavity, one in the E-plane and another in the H-plane. Additional steps within the cavity may be included in order to more closely approach the fundamental mode.
A method of transforming an electromagnetic signal between the fundamental mode of a standard rectangular waveguide at one end of a waveguide adapter having an internal cavity bounded by a wave-confining structure to a field distribution at the opposite end of the adapter that is desirable for a quasi-optic grid array assembly, is disclosed. The method comprises adjusting the size of internal walls of the adapter with at least one discontinuous step in the E-plane at a predetermined distance from the waveguide, and adjusting the size of internal wall of the adapter with at least one spatial discontinuity in the H-plane at a predetermined distance from the waveguide. The method may further include the steps of providing a grid array assembly, having a grid array and dielectric bounding the array, at the opposite end of the adapter and adjusting the normal distance between the edge of the grid array and the adapter wall at the opposite end to further determine the field distribution on the grid array.